This invention relates to a new class of steel alloys especially useful for the manufacture of case hardened gears and other products made from case carburized steel alloys.
Currently, there are a number of high performance gear and bearing steels on the market. A number of these materials utilize primary carbides to achieve their high surface hardness and others use stage one or stage three tempered conditions with epsilon carbide or cementite strengthening. Primary carbides are formed when the carbon content exceeds the solubility limit during the solution treatment and large alloy carbides precipitate. This is the case in particular for secondary hardening steels using alloy carbide strengthening for greater thermal stability to improve properties such as scoring resistance. However, research indicates that primary carbide formation can have a detrimental impact on both bending and contact fatigue resistance. Formation of primary carbides can also make process control difficult for avoidance of undesirable carbide distributions such as networks. In addition, primary carbide formation in current gear and bearing steel can lead to a reversal in the beneficial residual compressive stresses at the surface. This is due to a reversal of the spatial distribution of the martensite start temperature due to the consumption of austenite stabilizing elements by the primary carbides. Thus, there has developed a need for case hardenable steel alloys which do not rely upon primary carbide formation, but provide secondary hardening behavior for superior thermal stability. This invention provides a new class of steel meeting this requirement, while exploiting more efficient secondary hardening behavior to allow higher surface hardness levels for even greater improvements in fatigue and wear resistance.
In applications of sliding wear the formation of primary carbides can be beneficial; however, in current gear and bearing steels this can lead to a reversal in the beneficial residual compressive stresses at the surface due to the consumption of elements promoting hardenability by the primary carbides.
Thus, there has developed a need for case hardenable steel alloys which do not rely upon primary carbide formation.
Briefly, the present invention comprises a class of case hardenable steel alloys with carbon content in the range of about 0.05 weight percent to about 0.24 weight percent in combination with a mixture of about 15 to 28 weight percent cobalt, 1.5 to 9.5 weight percent nickel, 3.5 to 9.0 weight percent chromium, up to 3.5 weight percent molybdenum, and up to 0.2 weight percent vanadium.
The microstructural features are a Nixe2x80x94Co lath martensite matrix steel strengthened by M2C carbides typically containing Cr, Mo and V. Typical processing of this class of steels includes case carburizing, solution treatment, quenching, and tempering, although due to the high alloy content, quenching may not be required. Case carburizing produces a gradient in the volume fraction of the M2C carbides and results in a concomitant increase in hardness and promotes a surface residual compressive stress. The efficiency of the M2C strengthening response allows this class of steels to achieve very high surface hardnesses with limited soluble carbon content. Thus, this class of steels have the ability to achieve very high surface hardnesses without the formation of primary carbides.
Typical advantages of this class of alloys include ultrahigh case hardness leading to superior wear and fatigue resistance, superior core strength and toughness properties, optional air hardening resulting in less distortion, and higher thermal resistance.
This new class of secondary hardening gear and bearing steels are matrix steels utilizing an efficient M2C precipitate strengthening dispersion. Because of the efficiency of this strengthening dispersion, a superior combination of properties can be attained for a given application. For example, in situations where the desired surface properties are similar to current materials, the core strength and toughness can be superior. In applications where superior surface properties are desired, the disclosed steels can easily outperform typical materials while maintaining normal core properties, and in applications which require corrosion resistance, these new steels can provide stainless properties with surface mechanical properties similar to typical non-stainless grades.
These and other objects, advantages and features of the invention will be set forth in the detailed description which follows.